The proposed research aims to clarify the mechanisms involved in the pathogenesis of the virus induced spontaneously occurring leukemia of AKR mice. Recent experiments have demonstrated genetic changes in the AKR endogenous viruses, with the generation of recombinant variants, prior to the development of overt leukemia. Furthermore, a virus has been isolated from an AKR leukemia cell line (AKT8) that transforms mink firbroblasts in vitro. These results have led to the hypothesis that the final step in leukemogenesis is the generation of a recombinant viral genome capable of direct oncogenic transformation. Characterization of ADT8 virus, and testing of the above hypothesis, are the major objectives of this proposal. The in vivo activity of AKT8 virus will be tested by inoculation of young AKR mice with a variety of pseudotypes (AKT8 is replication defective). In vitro transforming activity will be tested with primary cultures of mouse spleen, thymus, bone marrow and fetal liver cells, in both liquid and agar based tissue cultures. Antigens common to AKR leukemia cells and fibroblasts non-productively transformed by AKT8 will be evaluated with sera by antibody and complemet mediated cytotoxicity, and by transplantation testing. The genetic origin of AKT8 will be investigated by restriction endonuclease analysis of transformed xenogeneic cell DNA, with comparison made to the same xenogenic cells infected with AKR endogenous viruses. Further experiments will evaluate the activity of AKT8 and non-transforming recombinant viruses in thymus-ablated mice, and approach the enhanced leukemogenicity of some of these isolates.